The electrical properties of semiconductor substrates such as GaAs are a sensitive function of the number and distribution of dislocations in the material. Semi-insulating GaAs grown by the liquid encapsulated Czochralski method is emerging as a viable substrate for producing complex devices and integrated circuits. The number of dislocations in such material is an important parameter. Many GaAs users require substrates with a dislocation density lower than some specific limit. It therefore is of importance to be able to measure the dislocation density in the material For certain purposes, the spatial distribution of dislocations in the material is important, requiring an ability to map the variation of dislocation density through the material.
It is known in the art that dislocations in GaAs may be revealed by etching the sample with a suitable chemical. Dislocations that intersect the surface of the material are revealed as etch pits after etching. Typical etch pits formed by potassium hydroxide etching of GaAs are shown in FIG. 1. The number of such pits per unit surface area, or etch pit density, is a reliable indicator of the number of dislocations within the substrate. The pits assume a pattern reflecting slip planes in the crystal. (FIG. 2)
Etch pit density is commonly measured by visual pit counting under the microscope. The technique, as usually practiced, involves visual observation of a relatively small number of areas (say, nine) on the wafer and manual counting by a human operator. The technique is slow and inaccurate, principally due to fatigue of the operator.
A technique for measuring GaAs etch pit density (EPD) by monitoring optical reflectivity through measurement of the time required to get a standard exposure in a photomicrograph of the surface is known. [Dobrilla, Mat. Letters 3 (7,8) May 1985, 299] The technique is based on the principle that pits formed on GaAs by potassium hydroxide etching are substantially less reflective than the surrounding surface. Accordingly, the intensity of light reflected to the film in a camera equipped microscope can be approximated by a decreasing, linear function of the number of pits in the field of view of the objective. Consequently, the time required to get a correct exposure is proportional to the local, non-uniform etch pit density. A map of the etch pit density and a quantitative measurement of EPD at specific points can be obtained by shooting pairs of microphotographs with differing exposure times at each point.
Rees, Stirland and Bicknell [Mat. Lttrs. 4 (11,12) October 1986, 455] have shown that EPD is related to the fraction FW.sub.c of surface covered by etch pits and the area of each pit A.sub.p by EQU EPD=-(1/A.sub.p) log (1-FW.sub.c).
A system for determining EPD by using a Vidicon camera to recognize etch pits has been described by Toyoda, Aota and Takahashi, in Defect Recognition and Image Processing: Montpellier, 1985, ed. J. P. Fillard (Elsevier, Amsterdam, 1985), 141. The system has a low spatial resolution and the measurement is much slower than the one here proposed.
It is old in the art to measure surface properties of materials by methods employing reflected or scattered light. Schmidt, U.S. Pat. No. 4,194,127, teaches an optical system for detecting and automatically plotting surface defects on polished single crystal semiconductor substrate wafers. The system employs an optical microscope to produce an image of the surface. The image is blurred by a translucent material and the light transmitted by the translucent material is then sensed by a light sensitive instrument such as a photomultiplier tube. Defects in the surface of the wafer appear bright in the microscope in contrast to the background. When the microscope is focussed on a defect, the output of the photomultiplier tube increases. The photomultiplier tube signal is amplified and fed to a plotter. A map of the defects is produced by moving the wafer-bearing microscope stage in the two orthogonal transverse directions in a stepwise manner.
Divens, et al. and Yoshikawa, et al. measure surface features such as photoresist lines, edges, cracks and pinholes by means of reflected laser beams as described in Divens, et al., U.S. Pat. No. 4,656,358 and Yoshikawa, et al., U.S. Pat. No. 4,505,585. The method of Divens, et al. involves focussing an ultraviolet laser beam on the surface and detecting the scattered and reflected light. The optical system includes a high numerical aperture objective close to the sample surface, two UV optical trains, and a UV detector. The sample is transversely moved under a stationary microspot provided by a tightly-focussed UV laser beam. The scattered and reflected light passes back through the objective of the microscope, through the second optical train to the UV detector. The UV detector provides a signal representative of the intensity of the scattered or reflected light. Plots of the intensity of reflected light as a function of transverse position of the sample provide accurate and reproducible profiles of the features being measured.
Yoshikawa et al. employ a system comprising a turntable on which the wafer is mounted, a laser beam directed onto the surface, a photo detector for detecting the light reflected from the surface, and electronics and a data processor to generate and record a defect signal when the laser beam falls on a surface defect.
An apparatus for examining and detecting macroscopic defects in the surfaces of polished semiconductor substrates is disclosed by Kugimiya, U.S. Pat. No. 4,547,073. The apparatus comprises a light source, a first optical means for converging the light to a parallel beam and projecting it onto the surface, and a second optical means for transporting the light reflected by the surface to a light receiving screen where millimeter sized defects are detected by specific shading patterns, lines, stripes or dots.
Methods of measuring the roughness of surfaces are taught by Jakeman et al., U.S. Pat. No. 3,971,956, and Dandliker et al., U.S. Pat. No. 3,922,093, wherein light is shone on the surface and the radiation scattered or reflected at oblique angles is detected and analyzed.
Yet another method of measurement of surface roughness, and amorphism, of polycrystalline silicon films is described in Harbeke, U.S. Pat. No. 4,511,800. The method involves observing the differential optical reflectance of a test surface and a surface of known minimal roughness. The respective intensities of the two reflected beams are detected and measured. The difference in intensities of the two reflected beams can be related to the surface roughness of the sample being examined. The device employs optical trains to focus one beam on the sample and the reflected beam on the detector, a beam chopper driven by a motor, control electronics, including a lock-in amplifier, and a logic unit for calculating the reflectance signal. The method measures the root mean square roughness of amorphous or polycrystalline semiconductor materials. Since there is no direct relation between surface roughness and etch pit density, the method is not capable of measuring etch pit density, nor is the method capable of measuring surface properties of single-crystal layers. The method would be overloaded by an attempted measurement of etch pit density.
All the prior art described above is capable only of revealing relatively rare defects on an otherwise perfect surface, measuring the departure of the sample from perfection. The present invention is intended to provide a means of reliably measuring a very large number of defects on a sample, namely thousands of etch pits.